As beautiful as they are, dahlias are not without their problems. Here are some issues that might plague your plants.
Dahlia mosaic virus
Mosaic virus appears as yellow veining in the leaves, which are quite distinct looking compared to healthy green leaves. It looks much like spider mite damage, but without the browning on the leaves. Also, mosaic virus affects the entire plant, including new growth, not just a few leaves on the plant. If your plant has true mosaic, remove the unhealthy plant from your garden to prevent the virus from spreading to other plants.
As the summer heats up, you might have issues with these little guys. They are tiny (you likely won’t even see them), but you will see the damage they cause. The plant will appear to be drying up, and the leaves will have brown edges and yellowing along the veins of the leaves (you may notice webbing too). The browning and damage appear at the bottom leaves, and as time goes on, they will work their way up the plant. Spider mites attach to the back side of the leaves and feed off the sap from the veins. As the leaf dies from their feeding off of it, they move on up to the next leaves. If you don’t treat the plant, they will eventually kill it. Any spray you use must have a miticide in it, or it will not stop them.
This is a grayish white powder-residue on the leaves, and it appears later in the growing season. A fungicide should be applied before the problem takes hold. Begin applying fungicide in midsummer, and reapply every two weeks.
Aphids, thrips, and beetles
All of these show up in late spring and can be controlled with organic insecticidal sprays. Usually you’ll see the insect first (aphids pictured), but the damage can look like pinholes in the leaves or a yellowing of the entire leaf.
Snails and slugs
These pests are a problem early when the plants are pushing through the soil. They love chewing on the tender new shoots, so slug baits are a must.
To learn more about dahlias, check out Out-of-the-Ordinary Dahlias.
Nicholas Gitts is a second-generation farmer and owner of Swan Island Dahlias in Canby, Oregon.